Do you think the roof has done this all its life or just since the new shingles got installed? I dont think someone would have put up with this problem for 30+ years.
Did you see the old roof before the new roof was put on?
What shape was the old roof in?
Was the old roof competely torn off or did the owner go over top?
Do you see any repair areas in the drywall where it is currently leaking now?
Did the previous owner provide you with a disclosure sheet on the condition of the house?
Are there any mold areas in the attic that would indicate that the leaks where there prior to your purchase of the home?
Is there new wood installed on the roof where it is now leaking?